This invention relates to a heat insulator for pipe lines for transportation of low temperature fluids such as LPG, LEG, LNG and the like liquefied gases. More particularly, this invention relates to an improvement in a heat insulator for pipe lines for transportation of low temperature fluids, and comprises a plurality of longitudinally divided hollow cylindrical heat-insulating units each composed of a heat-insulating panel element of a circular arc in cross section made of a foamed synthetic resin and a metal protective cover (a metal jacket) overlaying the outer surface of the element and integrally combined therewith.
In past conventional heat insulation systems for pipe lines of this type, rigid polyurethane foam shaped into a circular arc panel was widely used as a heat-insulating unit. Because of its excellent workability and heat-insulating property, a heat insulator made of rigid polyurethane foam was considered to be suitable for the purpose of heat-insulating pipe lines for transporting a supercooled fluid kept at a temperature as low as -100.degree..about.-200.degree. C.
A practical method for constructing such heat insulation system comprises assembling a plurality of the heat-insulating panel elements with a circular arc cross section to form cylindrical heat-insulating elements arranged in series on the outer periphery of a pipe within which a supercooled fluid is conveyed, applying, if necessary, a joint sealer to the joint contact surfaces of the adjacent elements, fixing the assembled heat-insulating elements around the pipe by a proper fixing means such as an adhesive, an adhesive tape, wires, or steel bands, applying a layer of a water- and moisture-proof material such as mastic or a water-repellent sheet onto the surface of the heat-insulating elements, and winding a thin metal plate such as a zinc- or resin-coated iron plate on the layer for ensuring sheathing and moisture-proof purposes.
However, the heat-insulating system of this type is disadvantageous in that it requires hard labor and incurs considerable costs and time for construction works. In order to overcome such defects, an attempt was made to simplify the construction works for the heat-insulating system by previously binding a thin metal plate onto the outer surface of the heat-insulating panel element of a circular arc in cross section made of a foamed synthetic resin with the aid of an adhesive or by utilizing self-adhesiveness of the foamed synthetic resin. A thin metal plate exhibits very excellent performance as a moisture-proof and sheathing material in the heat-insulating system, but the metal is significantly different in expansion coefficient from the foamed synthetic resin. Such difference permits partial delamination of the thin metal plate from the heat-insulating element made of the foamed synthetic resin, thus forming wrinkles on the metal plate due to the stress caused by shrinkage of the foamed synthetic resin at a low temperature during the operation of transporting a supercooled fluid through the pipe line. In case such wrinkles are formed in the joint contact surfaces (referred to hereinafter simply as the joint portion) between the adjacent cylindrical heat insulating units, moisture and water will enter through the wrinkled joint portions to deteriorate the heat-insulating property and, in the extreme case, breakage of the insulator will occur by increase in volume of the entered water by freezing, thus resulting in such disadvantage that the function as heat-insulating system be seriously damaged to incur a considerable loss.
Since the foamed synthetic resin is greater in the degree of shrinkage at a low temperature than the transportation pipe, interstices or voids will be formed in the joint portion between the adjacent heat-insulating units, especially in case of pipe lines being kept at an extremely low temperature, whereby permeation of moisture and water as well as thermal short circuit will occur in the damaged joint portions, and what is more, breakage or rupture of the insulator itself will take place on freezing of the permeated water.
Proposed to overcome such drawbacks is a method wherein a refractory coating material, such as foamed glass, heat-shielding material made of calcium silicate or pearlite or asbestos plate, previously processed to have a given shape is mounted to the outside of a foamed synthetic resin insulating element at the same time or after application of the foamed synthetic resin insulating material and finally a thin metal plate as the outermost jacket is applied for the purpose of protecting the refractory material and adding beautiful appearance (Japanese Patent Publn. No. 17871/68 and Japanese U.M. Publn. No. 4386/71). However, this method still involves such drawbacks that many troublesome steps are needed for previously shaping the refractory coating material and conveying the shaped article to the place where a heat insulator for pipe lines is constructed and that since such refractory material is brittle, loss of the material caused by mechanical damage during the transportation and construction of insulator is not negligible and incurs high costs. Thus, an improved refractory coating material is proposed to overcome such drawbacks wherein the previously shaped pearlite refractory material is bound with a double adhesive tape (Japanese U.M. Publn. No. 10391/71 ). However, this refractory coating material also involves such technical and economical disadvantages that it is practically impossible to bind the pearlite refractory material and the foamed synthetic resin insulating material with an adhesive tape so as to exert a binding strength strong enough for handling, that even if both materials are bound temporarily, both materials will be separated while being conveyed or used for construction of the insulator, and that further application of a moisture-preventing material and a top-covering material is needed.
In the field of heat insulators for pipe lines, therefore, there is a great demand for developing a heat insulator not only improved in moisture-proof and heat-insulating properties but also prevented from delamination due to shrinkage of the foamed synthetic resin at low temperatures.